Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hayes!tnixon From: tnixon@hayes.uucp Newsgroups: comp.dcom.modems Subject: Re: help wrt UUCP between 3B1 OBM and TB Message-ID: <3938.282a991a@hayes.uucp> Date: 10 May 91 12:59:06 GMT References: <29.28280170@zswamp.uucp> Organization: Hayes Microcomputer Products, Norcross, GA Lines: 93 In article <29.28280170@zswamp.uucp>, root@zswamp.uucp (Geoffrey Welsh) writes: > In a letter to All, Greg Andrews (gandrews@netcom.COM ) wrote: > > >However: Stop bit deletion is allowed in the 'specification' > >(actually the correct term is recommendation) for V.22bis, > > Hmmm, delete the stop bit every 8th character and you have a 1.3% > throughput improvement... hardly seems worth the hassle. > > I suppose that the receiver modem should be able to tell if the stop bit > is omitted, since a space will appear when a mark was expected... doing this > every character would result in an 11.1% throughput improvement (about half > of that achieved with MNP3&4 or LAP-M), for a maximum 2400 bps throughput of > 267 CPS! There seems to be some confusion here regarding stop bit deletion. CCITT V.42 (error control) deletes the start AND stop bits from EVERY character. Only the 8 data bits are transmitted, with blocks being framed with flags, frame headers, and FCS. The frames are transmitted synchronously between the modems, perfectly aligned to the carrier clock speed. The speed used on the DTE-DCE interface is independent of the line speed, and buffering and flow control are used to align the throughput on one interface to the other. What Greg is talking about is completely different -- CCITT V.14 async-to-sync conversion. V.14 is referenced by CCITT V.22, V.22bis, V.26ter, V.32, and V.32bis, and is also used in Bell 212. V.14 is for NON-error-control communications. It allows the modem to compensate for the fact that the DCE-DTE interface is asynchronous, that the clocks on the DCE and DTE may be slightly off. In V.14's "basic range", the DTE may be up to 1% "too fast" (meaning, for example, it can actually be sending up to 2424bps on a 2400bps modem), or as low as 2.5% too slow (as little as 2340bps). In the "extended range", the lower limit of -2.5% is the same, but the upper limit ("too fast") is extended to +2.3% (as high as 2455bps). If the DTE is running "too slow", the V.14 async-to-sync conversion process simply adds an additional stop bit every few characters, as necessary. That's pretty simple! The remote modem in this case doesn't do anything, really; it just delivers all the bits. If the DTE is running "too fast", and is sending lots of characters one after the other, then the DCE has a problem; it can't put that many bits onto the line, because it can only send one bit per clock cycle. So, it occassionally DELETES a stop bit (always value 1), and just starts the next character (with a start bit, always value 0). It is only permitted to do this once every eight (basic range) or every four (extended range) characters. The receiver detects that there is a start bit where there is supposed to be a stop bit, and re-inserts the stop bit. This, of course, means that the receiving DCE is having to "run fast", too, putting out more bits to its DTE than it normally would. Most DCEs accomplish this by shaving a little bit off EVERY stop bit they send to the DTE -- sending 3/4ths of a stop bit if they support the "extended" range, or 7/8ths of a stop bit if they support the "basic" range. That's fine, because the DTE samples bits at 1/2 of the time into the bit. If there's not an additional character to send, then, of course, the DCE just extends the stop bit (the usual case). The other consideration is with BREAK signalling. For this process to work, breaks must be long enough that the receiving DCE can distinguish them from two consecutive null characters with the stop bit deleted on the first one. V.14 enforces this by saying that breaks must be at least 19 bits long (assuming 8-bit characters). This "appears" to receiving modem as though the stop bits had been deleted from two consecutive characters, which is not permitted, so it doesn't re-insert the stop bits and allows "break" to pass through to the DTE unmolested. The transmitting DTE must insure that once a break is started by the DTE, that it is forced to be at least 19 bits long; otherwise, the receiving DCE will "fix it" by inserting a stop bit in the middle. V.14 is implemented in all modems by all manufacturers that support async transmission on V.22, V.22bis, V.26ter, V.32, and V.32bis; it's not a special feature (or bug!) in Telebit modems. It's not done to try to improve throughput, but to account for the inaccurate bit rate clocks used in many async terminals, PCs, and computers. -- Toby -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net